The present invention generally concerns cold forging and the invention concerns more specially a threaded mandrel for the cold forging of internal threads in a tube or cylinder.
Internal threads in tubes or cylinders have earlier been produced by cutting methods such as turning, shaping, slotting etc. The production of internal threads in tubes and cylinders etc. has recently also been accomplished by cold forging, whereby a tube or cylinder is forged around a mandrel which, after the process, is removed from the tube or cylinder.
Cold forging involves several advantages compared to cutting methods. By cold forging, a finer surface finish can be achieved than with cutting methods, the material becomes harder as a result of the fact that metal fibers are not cut off and that a certain packing of the material is gained by the mechanical working, internal stresses from earlier handling are eliminated, the threads can be formed to extremely fine tolerances and cold forged products are produced with an even and high quality etc.
Cold forging around a mandrel however does present some problems, specially the problem of removing the mandrel after the cold forging process. By cold forging the workpiece is pressed so hard against the mandrel that material displacement occurs in the workpiece and the material is pressed into the mandrel's threads or other various shaped pits. Depending on the material's elasticity, the tube or cylinder and the mandrel will be radially pressed against each other very hard and this makes it very difficult to remove the mandrel from the tube or cylinder. Cold forging is usually effected by a progressive working of the tube or cylinder by using forging hammers which are run over the workpiece from one end to the other. During this hammering the tube or cylinder will lengthen in a direction which is opposite the direction of movement of the workpiece, and this lengthening of the material is the cause of axial loads which arise between the forged workpiece and the mandrel and which loads make it even more difficult to release the mandrel from the workpiece.
The invention is intended to eliminate these problems and to provide a mandrel for the production of internal threads in a tube or cylinder by cold forging a forging blank around the mandrel.